Method and equipment for controlling properties of paper

ABSTRACT

The invention relates to a method and equipment for controlling properties of paper, which method comprises modelling the effect of the flow of retention agent (F ra ) both on the amount of filler in the white water and on the paper ash content (ASH), and the effect of the flow of filler (F fi ) both on the paper ash content (ASH) and on the amount of filler in the white water. The fulfilled modelling is used to adjust the amount of filler in the white water and the paper ash content (ASH) by controlling simultaneously the flow of retention agent (F ra ) and the flow of filler (F fi ).

FIELD OF THE INVENTION

The invention relates to a method for controlling properties of paper,which method comprises the steps of measuring a variable describing theamount of filler in the white water and the paper ash content and/or theheadbox ash content, and adjusting the amount of filler in the whitewater by controlling the flow of retention agent on the basis of themeasurements of the variable describing the amount of filler in thewhite water, and adjusting the paper ash content by controlling the flowof filler on the basis of the measurement result of the paper ashcontent and/or the headbox ash content.

The invention also relates to equipment for controlling properties ofpaper, comprising means for measuring a variable describing the amountof filler in the white water and the paper ash content and/or theheadbox ash content, and control means for controlling the flow ofretention agent in order to adjust the amount of filler in the whitewater on the basis of the measurement result of the variable describingthe amount of filler in the white water, and for controlling the flow offiller in order to adjust the paper ash content on the basis of themeasurement result of the paper ash content and/or the headbox ashcontent.

BACKGROUND

As regards controls in so-called short circulation of a paper machine,paper properties are controlled at present for example by adjusting thewhite water total consistency through adjustment of the flow rate of theretention agent. In such a case, the result of the measurement of thewhite water consistency forms the basis for controlling a retentionagent pump or valve in order to adjust the flow rate of the retentionagent to be supplied. Further, in the prior art the ash content of paperis controlled by measuring the ash content for example from the finishedpaper with a measuring beam and/or by measuring the ash content in theheadbox and/or the ash content of the stock and by controlling the flowrate of the filler to be added to the stock. The basis weight of thepaper is controlled by means of stock flow control, which also takesinto account changes in the stock consistency on the basis of totalheadbox consistency and/or a measurement result obtained from themeasuring beam on the basis weight of the paper. Each control operatesindependently regardless of the other controls. Such an arrangement isdisclosed for example in the reference “Lyhyen kierron sakeussäädöt,AEL/INSKO P906202/96 V, Nokelainen J., 1996” (consistency controls inthe short circulation). Controlling one property also affects otherproperties; for example variation in the amount of retention agentand/or filler affects the basis weight, and therefore one or morecontrols are adjusted to operate so slowly that they do not interferewith the faster controls. Such a slow control cannot naturallycompensate for rapid changes occurring in the property it controls. Onthe other hand, for example during grade changes the controls arecarried out at consecutive stages and therefore the total time requiredfor the changes is rather long. The situation is especially difficultwith paper grades having a considerable ash content or with paper gradesutilizing coated broke, which means that the papermaking process will besubjected to problems with the ash due to variation in the amount ofbroke or in the ash content of the broke, since in such a case changesin the amount of retention agent strongly affect the paper ash contentand, correspondingly, changes in the amount of filler strongly affectthe white water consistency.

SUMMARY OF THE INVENTION

The purpose of the present invention is to provide a method andequipment for rapid and effective control of paper properties.

The method according to the invention is characterized by modelling theeffect of the flow of retention agent on the amount of filler in thewhite water and on the paper ash content, and the effect of the flow offiller on the paper ash content and on the amount of filler in the whitewater, and adjusting, on the basis of said modelling, the amount offiller in the white water and the paper ash content by simultaneouslycontrolling the flow of retention agent and the flow of filler.

Further, the equipment according to the invention is characterized inthat the equipment comprises a model of the effect of the flow ofretention agent on the amount of filler in the white water and on thepaper ash content, and of the effect of the flow of filler on the paperash content and on the amount of filler in the white water, and that thecontrol means comprise means for controlling simultaneously the flow ofretention agent and the flow of filler in order to adjust the amount offiller in the white water and the paper ash content on the basis of saidmodel.

The invention is based on the idea of modelling the effect of the amountof retention agent both on the variable describing the amount of fillerin the white water and on the paper ash content, and the effect of theamount of filler both on the paper ash content and on the variabledescribing the amount of filler in the white water, and the executedmodelling is used to simultaneously adjust, by means of the amount ofretention agent, the amount of filler in the white water and the paperash content, and by means of the amount of filler the paper ash contentand the amount of filler in the white water. The idea of a preferredembodiment is to model the effect of the amounts of retention agent andfiller on the basis weight of the paper and to control the basis weightof the paper by means of the aforementioned amounts by controllingsimultaneously the basis weight by means of the stock flow.

In connection with the present application, the term “paper” also refersto board in addition to paper.

The invention has the advantage that it provides faster and moreeffective control of paper properties in paper machine short circulationthan the previous methods.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is described in more detail in the accompanying drawings,in which

FIG. 1 shows schematically a papermaking process, and

FIG. 2 shows a diagram of the structure for optimization according tothe invention.

DETAILED DESCRIPTION

FIG. 1 shows schematically a papermaking process. Stock is supplied to apaper machine via a wire pit silo 1. Water is mixed into the stockarriving from the wire pit silo 1 in order to adjust the consistency toa suitable level. The stock is thereafter supplied to a headbox 2, fromwhich the stock is supplied to a former section 3, where it is formedinto a fibre web 4. The fibre web 4 is dried in a dryer section 5, whichis followed by a measuring beam 6. The paper machine also comprises forexample a press section and a reel, and it may also comprise e.g. sizepresses or a calender, which are not shown in the accompanying figurefor the sake of clarity. Furthermore, the operation of a paper machineis known per se to a person skilled in the art and therefore it will notbe described in greater detail in this connection.

For the control of paper properties according to the invention, thebasis weight BW and the ash content ASH of the paper are measured by themeasuring beam 6. The variable “paper ash content ASH” may be forexample the proportion of ash to the basis weight or dry weight of thepaper or the amount of ash, i.e. the mass flow, in a time unit. Papermoisture Moi can also be measured by the measuring beam 6 if the totalash content is to be determined, for example. Further, headbox ashcontent ASH_(hb) and white water total consistency CS_(ww) are measured.Instead of the white water total consistency CS_(ww)it is possible tomeasure some other variable that describes the filler content of whitewater, such as the consistency of the bottom, top, inner or outer whitewater or for example the white water ash content. However, measurementof the white water total consistency CS_(ww) is easy and simple.Disturbance variables that are measured for the purpose of optimizationinclude the machine speed S and the stock ash content ASH_(ts). Themachine speed S can be measured from either one or several points, forexample from the former section 3 or the reel or from both points.Instead of the stock ash content ASH_(ts) it is possible to use a valuecalculated for the mass flow of the stock ash content QA_(ts), such that

QA_(ts)=F_(ts)*CS_(ts)*ASH_(ts),

wherein

QA_(ts) is the mass flow of the stock ash content,

F_(ts) is the stock flow

CS_(ts) is the total consistency of stock, and

ASH_(ts) is the stock ash content.

The total consistency of stock CS_(ts) is usually standardized with aseparate control provided before a machine chest 7, but in the optimumcontrol according to the invention the total consistency of stockCS_(ts), is also introduced into the process as a disturbance variable.

According to the invention, the flow of retention agent F_(ra) iscontrolled by a flow regulator. Retention agents increase the retentionof fines and fillers and simultaneously speed up drainage in a mannerknown per se. Retention agents may be inorganic retention agents,natural organic retention agents or synthetic water-soluble organicpolymers in a manner known per se. Further, the flow of filler F_(fi) iscontrolled by a flow regulator. The purpose of a filler is, among otherthings, to improve paper formation, surface properties, opacity,brightness and printability and to decrease manufacturing costs. Afiller may be, for example, kaolin, calcium carbonate, titanium dioxideor talc in a manner known per se.

If desired, the control system according to the invention can be used tosimultaneously adjust the stock flow F_(ts) with a separate flowregulator. A flow regulator that is used to control the flow ofretention agent or filler or the stock flow can be for example a valveor a use-controlled pump or both. Flow adjustment and control refer inthe present application specifically to controlling the flow rate, whichcan be denoted for example in the following manners that are known perse: l/min or g/ton of production.

If there is no control of the flow of filler F_(fi), the flow ofconstant filler F_(fic) to be supplied to a mixing tank 8 can becontrolled instead. However, in such a case it is necessary to take intoaccount an additional time constant and therefore the accuracy of thefinal control may not necessarily be very good.

FIG. 2 shows a structure of optimizing the control arrangement accordingto the invention. Parameters of process models include the necessarycoefficients and time constants, which have been determined by utilizingboth knowledge obtained from designing a paper machine and process testscarried out at different operating points. Models used by a predictormay be different from those used for the optimization. The predictor maycalculate a new model for the optimization during each round ofexecution, and the model takes into account changes occurring in thespeed S and the production rate PS_(ts) and changes that will take placein the future and that may be known in advance for example during agrade change. Determining a model is known per se to a person skilled inthe art and therefore it is not discussed in greater detail in thisconnection. The predictor receives as input a disturbance variable thatis the machine speed S, and the predictor takes it into account in caseof change and provides for optimization a model which is in a requiredform and which takes into account the change in the speed. Thedisturbance variable may also be the production rate PS_(ts), in whichcase

PS_(ts)=F_(ts)*CS_(ts),

wherein

PS_(ts) is the production rate,

F_(ts) is the stock flow, and

C_(ts) is the total consistency of stock.

During optimization the controls are optimized based on the models,separate stored controls, measurements, disturbance variables andrestrictions.

The optimization is a block with rather simple operation utilizing themodels that are produced by the predictor and that describe for examplethe effect of a change in the flow of retention agent and filler on thepaper ash content and on the white water total consistency. Thepredictor provides a prediction and produces a new process model for theoptimization. The predictor contains diverse functions and takes intoaccount different situations and changes therein from various aspects.For example, the predictor takes into account the effect of variation inthe machine speed and/or the draw on the basis weight of the paper.Further, a part of the web edge is typically cut off and returned to thewire pit silo, and if changes occur in the size of the section that wascut off, the predictor takes into account and predicts the resultingchange in the white water total consistency.

The target values include the basis weight SPBW, the paper ash contentSPASH, the headbox ash content SPASH_(hb) and the white water totalconsistency SPCS_(ww).

A process model is a dynamic model having as input variables the stockflow F_(ts), the consistency of stock CS_(ts), the flow of retentionagent F_(ra), the flow of filler F_(fi), the speed of the paper machineS and possibly also the stock ash content ASH_(ts). Output variables ofthe dynamic process model include the white water total consistencyCS_(ww) and the paper ash content ASH and, if desired, the headbox ashcontent ASH_(hb) and the basis weight of the paper BW. Control variablesused in an optimal control include the flow of filler F_(fi) and theflow of retention agent F_(ra). In such a case, it is possible tocontrol simultaneously the flow of filler F_(fi) and the flow ofretention agent F_(ra), which means that the control is rapid andeffective. These flows can be compensated for by utilizing either themachine speed S or the production rate PS_(ts), if desired. It is alsopossible to use the stock flow F_(ts) simultaneously as a controlvariable so that the adjustment of different paper properties, includingthe basis weight BW, can be controlled very well. It is possible topredict, by means of a dynamic model, the future values of the outputvariables, based on the existing operating point and the previous valuesof the input variables if there are no new changes in the control. Anoperating point describes the operating point of the process, whichmeans that the process is run with certain values, for example values ofthe basis weight BW.

A model-based optimal control algorithm calculates a guide valuetrajectory for a control variable on the basis of the target valuetrajectory of the controls and the predicted output variables, and thisguide value trajectory guides the process optimally to the target valuesin the desired manner at each moment. This data is forwarded to anautomation system. An essential feature of the method used is that theoptimal control algorithm is independent of the dynamic model used, andduring each control round it is possible to use a dynamic model that isdetermined separately and the optimal control algorithm may utilizedifferent weighting coefficients in different situations during a runand in principle during each control round. Such an arrangement isparticularly important during grade changes where this type of operationmakes it possible to predict the situation at each moment.

In a normal situation, each of the four target values can be assigned aset value, but in the optimization the paper ash content ASH is weightedmore than the headbox ash content ASH_(hb). Alternatively, it ispossible to take into account the paper ash content ASH and to disregardthe headbox ash content ASH_(hb). During a break, the headbox ashcontent ASH_(hb) can be assigned a set value and the paper ash contentASH can be disregarded entirely. After the break, normal operation isresumed. On the other hand, during a break the paper ash content ASH canbe replaced with a value provided by the model during the break, andnormal operation that is based on measurements can be resumed after thebreak.

The drawings and the related description are only intended to illustratethe inventive idea. The details of the invention may vary within thescope of the claims.

What is claimed is:
 1. A method for controlling properties of paperproduced by a paper making machine, said method comprising supplyingpaper stock into a headbox, conveying the paper stock from the headboxto a former section in which a paper web is produced and from whichwhite paper is expelled, producing paper from the paper web, measuringproperties of the paper, combining the white water into the paper stock,the white water, the paper stock and the paper having respective amountsof ash content, measuring a variable describing the amount of the fillerin the white water and the ash content in the paper, in the headbox orboth, said method comprising a control system including a dynamic modeland control variables, said model relating the effect of the flowretention agent on the amount of filler in the white water and on thepaper ash content, and the effect of the flow of filler on the paper ashcontent and on the amount of filler in the white water and the paper ashcontent by simultaneously controlling the flow of retention agent andthe flow of filler, the control system comprising predictor andoptimization steps, in which the predictor step takes into account theopereating variables and said model and provides a prediction on thebasis thereof, and the optimization step utilizes the predictionproduced in the predictor step to optimize the control variables.
 2. Amethod according to claim 1, further comprising the steps of producingin said model the effect of the flow of retention agent, the flow offiller and stock flow on the basis weight of the paper, and adjustingthe basis weight based on the model by simultaneously controlling theflow of retention agent, the flow of filler and the stock flow.
 3. Amethod according to claim 2, wherein the model has as input variablesstock flow, consistency of stock, the flow of retention agent, the flowof filler and the speed of the paper machine, and as output variablesthe headbox ash content, white water consistency, basis weight of thepaper and the paper ash content, and as control variables the flow offiller, the flow of retention agent and the stock flow.
 4. A methodaccording to claim 3, wherein the input variables of the model alsoinclude the stock ash content.
 5. A method according to claim 1, whereinthe model has as input variables stock flow, consistency of the stock,the flow of retention agent, the flow of filler and speed of the papermachine, and as output variables white water consistency and the paperash content, and as control variables the flow of filler and the flow ofretention agent.
 6. A method according to claim 5, wherein the inputvariables of the model also include the stock ash content.
 7. A methodaccording to claim 1, wherein the predictor produces a new model for theoptimization during each control cycle.
 8. A method according to claim1, wherein, during a break, the model utilizes the value of the headboxash content.
 9. A method according to claim 1, wherein said predictorstep provides a dynamic model on the basis of the prediction. 10.Equipment for controlling properties of paper produced by a paper makingmachine comprising a headbox into which paper stock is supplied and fromwhich the paper stock is supplied to former section which produces apaper web from the stock, said web being supplied as paper to ameasuring beam, said former section having an outlet for white waterexpelled during production of the paper web, means for combining thewhite water into the paper stock, means for adding a flow of filler tothe paper stock, means for adding a flow of retention agent to the paperstocl, the white water, the paper stock and the paper having respectiveamounts of ash content, said equipment comprising means for measuring avariable relating the amount of filler in the white water, means formeasuring ash content in the paper, or ash content in the paper stock inthe headbox or both, a control system including a model and controlvariables relating the flow of retention agent on the amount of fillerin the white water and on the ash content of the paper, and of the flowof filler on the ash content of the paper and on the amount of filler inthe white water, means for controlling simultaneously, on the basis ofsaid model, the flow of retention agent and the flow of filler in orderto adjust the amount of filler in the white water and the ash content ofthe paper, the control system further including a predictor means and anoptimization means, the predictor means comprising means for taking intoaccount said model and operating variables for providing a predictionoutput, the optimization means comprising means for utilizing saidprediction output produced by the predictor means for optimizing thecontrol variables and the process of producing the paper.
 11. Equipmentaccording to claim 10, wherein said model further relating the effect ofthe flow of retention agent, the flow of filler and stock flow on basisweight of the paper, said control means further comprising means forsimultaneously controlling, based on said model, the flow of retentionagent, the flow of filler and the stock flow in order to adjust thebasis weight.
 12. Equipment according to claim 11, wherein said modelhas as input variables, the stock flow, the consistency of stock, theflow of retention agent, the flow of filler and speed of the papermachine, and as output variables the headbox ash content, the whitewater consistency, basis weight of the paper and the paper ash content,the model having as control variables the flow of filler, the flow ofretention agent and the stock flow.
 13. Equipment according to claim 12,wherein the input variables of the model also include the stock ashcontent.
 14. Equipment according to claim 10, wherein said model has asinput variables the stock flow, the consistency of stock, the flow ofretention agent, the flow of filler and a speed of the paper machine,and as output variables white water consistency and the paper ashcontent, the model having as control variables the flow of filler andthe flow of retention agent.
 15. Equipment according to claim 14,wherein the input variables of the model also include the stock ashcontent.
 16. Equipment according to claim 10, wherein said predictormeans includes means for producing a further model on the basis of theprediction output.